Continuous and discontinuous filament spinning technologies are known in art, and are commonly referred to as spunmelt technologies. Spunmelt technologies include both the meltblown or spunbond processes. A spunbond process involves supplying a molten polymer, which is then extruded under pressure through a large number of orifices in a plate known as a spinneret or die. The resulting continuous filaments are quenched and drawn by any of a number of methods, such as slot draw systems, attenuator guns, or Godet rolls. The continuous filaments are collected as a loose web upon a moving foraminous surface, such as a wire mesh conveyor belt. When more than one spinneret is used in line for the purpose of forming a multi-layered fabric, the subsequent webs are collected upon the uppermost surface of the previously formed web.
The melt-blown process is related to means of the spunbond process for forming a layer of a nonwoven fabric, wherein, a molten polymer is extruded under pressure through orifices in a spinneret or die. High velocity gas impinges upon and attenuates the filaments as they exit the die. The energy of this step is such that the formed filaments are greatly reduced in diameter and are fractured so that microfibers of indeterminate length are produced. This differs from the spunbond process whereby the continuity of the filaments is preserved.
Spunmelt equipment manufacturing companies, such as Reifenhäuser, Ason Neumag, Nordson, and Accurate Products have engineered numerous meltblown and/or spunbond manufacturing models that offer a variety of desirable attributes, such as increased polymer throughputs, better management of process air flow or polymer distribution, and improved control of filament deviations, to name a few. U.S. Pat. Nos. 4,708,619; 4,813,864; 4,820,142; 4,838,774; 5,087,186; 6,427,745; and 6,565,344, all of which are incorporated herein by reference, disclose examples of manufacturing equipment for the processing of spunbond or meltblown materials.
There is an increasing demand for articles produced from nonwoven containing sub-micron fibers. The diameters of sub-micron fibers are generally understood to be less than about 1000 nanometer (i.e., one micron). The sub-micron fibers webs are desired due to their high surface area, low pore size, and other characteristics. The sub-micron fibers can be produced by a variety of methods and from a variety of materials. Although several methods have been used, there are drawbacks to each of the methods and producing cost effective sub-micron fibers has been difficult. Conventional spunmelt equipment arrangements can not provide high quality, low defect fibers and webs that are predominantly microfine including submicron diameter fibers with narrow fiber size distributions.
Methods of producing sub-micron fibers include a class of methods described by melt fibrillation. Non limiting examples of melt fibrillation methods include melt blowing, melt fiber bursting, and melt film fibrillation. Methods of producing sub-micron fibers, not from melts, are film fibrillation, electro-spinning, and solution spinning. Other methods of producing sub-micron fibers include spinning a larger diameter bi-component fiber in an islands-in-the-sea, segmented pie, or other configuration where the fiber is then further processed so that sub-micron fibers result.
Melt fibrillation is a general class of making fibers defined in that one or more polymers are molten and extruded into many possible configurations (e.g. co-extrusion, homogeneous or bicomponent films or filaments) and then fibrillated or fiberized into filaments.
Melt film fibrillation is another method to produce fibers. A melt film is produced from the melt and then a fluid is used to form fibers from the melt film. Two examples of this method include Torobin's U.S. Pat. Nos. 6,315,806; 5,183,670; and 4,536,361; and Reneker's U.S. Pat. Nos. 6,382,526, 6,520,425 and 6,695,992, assigned to the University of Akron.
Electrospinning is a commonly used method of producing sub-micron fibers. In one manifestation of this method, a polymer is dissolved in a solvent and placed in a chamber sealed at one end with a small opening in a necked down portion at the other end. A high voltage potential is then applied between the polymer solution and a collector near the open end of the chamber. The production rates of this process are very slow and fibers are typically produced in small quantities. Another spinning technique for producing sub-micron fibers is solution or flash spinning which utilizes a solvent.
One of the ways to achieve high melt shear for a melt film fibrillation process is through high-speed gas (i.e., close to sonic or supersonic gas velocities). To obtain supersonic or transonic (close to sonic) velocities, flow typically needs to converge to a throat (narrowest part of the nozzle where the velocity reaches sonic levels) and then expands in a diverging section. Adiabatic nozzles (no heat gained or lost through the boundaries of the nozzle system) meeting these general criteria are known in the art, and include so-called Laval nozzles. Use of Laval type nozzles in fiber formation are disclosed, e.g., in U.S. Pat. Appln. Publ. No. 2004/0099981 A1, and U.S. Pat. Nos. 5,075,161 and 5,260,003. These methods utilize Laval nozzles to speed up the gas velocities to sonic and/or supersonic range. When polymer melt is exposed to such high gas velocities, it bursts into multiplicity of fine fibers. They generally use concentric input and channeling of gas and polymer melt in the discharge nozzles, which can be non-optimal from standpoints of equipment lay-out complexity and equipment maintenance, etc. However, other nozzle configurations, such as non-concentric (non-annular) layouts of nozzles pose challenges of their own. For instance, in a fiber or filament nozzle system where polymer melt and gas introduction proceed from separate side-by-side units, a problem tends to arise when the fiberization gas flows between a side with a surface or wall that is heated to a high temperature (e.g., due to polymer melt flow introduction from that side) and a surface or wall of an opposing side (for example, an gas introduction side) that is at a lower temperature than the polymer melt side. In such a scenario, gas flow tends to become unstable in the diverging section in prior nozzle configurations such as with a Laval nozzle. This leads to problems of lack of polymer shear, polymer back-flow or build-up into the gas side of the gas passage, and subsequently an unevenly varying excessive and diminishing polymer flow and fiberization. After sufficient melt build up occurs upstream into the gas side, polymer melt separates and is typically blown out as a “shot,” since the melt locally cools down and can no longer form fibers due to insufficient shearing of polymer. When the polymer flow starves as the other end of those variation, the shearing is excessive leading to undesired dust. New advances have been needed to allow production of consistently high quality sub-micron fibers for disposable articles in a more efficient manner at commercially-significant output levels.